The present invention relates to electrostatic spray coating apparatus, and in particular to an improved electrostatic spray coating apparatus having a material charging portion of significantly reduced capacitance and electrical energy storage capability.
Manual and automatic spray guns for mechanical atomization and electrostatic deposition of coating materials such as paint, lacquer, varnish and the like are known from such previously issued U.S. Patents such as U.S. Pat. Nos. 2,805,642; 3,048,498; 3,169,882; 3,169,883; 3,251,551; 3,253,782; 3,268,171; 3,583,632; and 3,737,099; etc. In general, the material is atomized from a nozzle by impinging streams of air on a stream of the material (air atomization), or by forcing the material through an appropriately shaped nozzle under adequate hydraulic pressure (hydrostatic atomization). Atomization may also be effected by centrifugal force and by electrostatic forces. As the coating material is atomized, an electrostatic charge is imparted to the material which causes the charged particles of atomized material to be attracted to work pieces or articles which are purposely grounded, whereby to cause most of the material to be deposited on the work.
The means for imparting an electrostatic charge to the material customarily includes a conductive charging electrode extending forwardly of the spray gun into the area in which atomization of the material occurs. Since a relatively high voltage at the electrode is used to impart the electrostatic charge, safety means ordinarily are provided to limit the voltage at the electrode should an operator of the gun come in contact therewith. Such a means may include a resistor connected in series between the charging electrode and a power supply for the gun. Should an operator of the gun come into physical contact with the electrode, the resistor limits the current which may be supplied to the electrode, thereby affording a measure of safety to the operator.
Despite the current limiting effects of such resistors, it nevertheless is possible under certain circumstances for a significant electrical charge to accumulate in the material charging portion of the spray gun. If, for example, the charging electrode of the gun is held in noncontacting proximity with an electrically conductive body, as a result of a corona generated about the charging electrode a charge will be accumulated on the body and may result in the discharge of an electrical arc between the body and the nozzle of the gun. Should such an arc occur in an explosive atmosphere, as may be created by an atomized mixture of certain coating materials in air, an explosion may result. Consequently, to minimize the possibility of the occurrence of such an electrical discharge or arc, the material charging portion of the spray gun desirably should exhibit minimum capacitance or minimum electrical energy storage capability.